Hi all, first post here.
I have an itch to get some tube warmth for my bass. I know lots of guys have done it, but here's where my plan may be different:
I have two (2) Fender Rumble "heads" that I want to use as the power amp, giving me a hybrid with a tube pre. They use the B&O Class D ICEpower modules. I know it can be done because Fender used this same approach on their TV series of bass amps (now discontinued).
I thought of making an Alembic F2B clone for the pre. The Rumbles I have are really combo chassis that I got off ebay (one is 150W, the other is 350W). There is lots of space inside for a pre and the power section stands alone so it should be relatively easy.
Anyway, where should I start? I have the schematics for the Rumbles, but no clue how to power a tube preamp.
My DIY experience is all with tube guitar amps, I've made 5 or 6 FWIW. Thanks in advance!!
I have an itch to get some tube warmth for my bass. I know lots of guys have done it, but here's where my plan may be different:
I have two (2) Fender Rumble "heads" that I want to use as the power amp, giving me a hybrid with a tube pre. They use the B&O Class D ICEpower modules. I know it can be done because Fender used this same approach on their TV series of bass amps (now discontinued).
I thought of making an Alembic F2B clone for the pre. The Rumbles I have are really combo chassis that I got off ebay (one is 150W, the other is 350W). There is lots of space inside for a pre and the power section stands alone so it should be relatively easy.
Anyway, where should I start? I have the schematics for the Rumbles, but no clue how to power a tube preamp.
My DIY experience is all with tube guitar amps, I've made 5 or 6 FWIW. Thanks in advance!!
You will need a supply for the tube heaters, 6.3 or 12.6 VAC and a low current B+ of around 300V. Hammond makes one or two small transformers that will do the job or you can use a pair of "filament" transformers where one is connected in reverse to create a 120V isolated winding to generate the 300VDC with a voltage doubler.
just take a look:
Dual Channel 1U bass/guitar preamp with low impedance transformer balanced output
cheers Uli
Dual Channel 1U bass/guitar preamp with low impedance transformer balanced output
cheers Uli
??
You've build 5 or 6 tube guitar amps, but don't know how to get started on a tube bass pre?? I don't get it. Same tubes, same voltages, different frequencies and input sensitivity.
The Alembic F2B (based on a Fender input stage) is a classic design and a fun schematic to start with.
You've build 5 or 6 tube guitar amps, but don't know how to get started on a tube bass pre?? I don't get it. Same tubes, same voltages, different frequencies and input sensitivity.
The Alembic F2B (based on a Fender input stage) is a classic design and a fun schematic to start with.
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Just to clarify this, there's no reason for it to be AC - there's many advantages in using DC instead - AC was used historically simply because it's easy to provide a suitable heater winding on the mains transformer.
Yeah, I built those guitar amps from schematics. This time I am trying to mate a tube pre (from scratch) with a SS power section (already made). I am really trying to "clone" a Fender TV series bassman (anyone have a schematic??).
I realize that the F2B can be done with its own transformers, but any idea if I can power it some other way from the mains? I think the ICEpower modules have an auxiliary tap, but I haven't looked into it yet.
One of my concerns is impedance matching of the tube output and SS input. Any wisdom for me?
Yes, impedance matching isn't required, and in almost all audio cases is a VERY bad thing anyway (the major exception been speaker matching on valve amps).
You simply want the load impedance (input of the SS amp) to be around five to ten times the source impedance (output of the valve preamp).
Adding a cathode follower at the output of the preamp should sort it out.
I would suggest a solid stage preamp input for your power amp
and the tube pre as a seperate preamp unit
I'm not familiar with the ICEPower modules, but I doubt they have a high voltage aux out. Tubes can be ran at lower voltages, but I prefer not to because of tone.
Turns out it's only 24 VDC and the stock SS pre uses it. Guess I'll go for the separate transformers... Then again, I have a Kingsley tube pedal (Jouster) which turns 12 VDC into at least 250 V internally on a small board. See it here:
Ah, the Tung-sol 12AX7. Nice tube. I like 'em a lot for bass! But that aside.
It looks like it has a boost converter (step-up SMPS) on the top side. That is an option to get the high voltage and in a sense replaces the needed transformer. But they're difficult to get right. Specially for audio. If you have no experience with them, then there's quite the learning curve. Personally, I prefer to keep it linear, but you might deem it worth an experiment. Maybe you can find a proven module. Or sacrifice the pedal 😀
It looks like it has a boost converter (step-up SMPS) on the top side. That is an option to get the high voltage and in a sense replaces the needed transformer. But they're difficult to get right. Specially for audio. If you have no experience with them, then there's quite the learning curve. Personally, I prefer to keep it linear, but you might deem it worth an experiment. Maybe you can find a proven module. Or sacrifice the pedal 😀
Haha -no way! It's incredible for guitar and more than decent for bass. The previous version used a toroidal transformer. Thanks for the pointers, everyone. I am going to sit down and figure out my approach. Will probably harass you with more questions as I go.
like I said, give your power amp a nice hi-z jfet input 😉
having the option of plugging directly into your power amp might be a nice and simple alternative when less power is needed
having the option of plugging directly into your power amp might be a nice and simple alternative when less power is needed
Hi again guys, I think I may go for the toroidal transformer approach for powering the tubes. They're small and not too expensive.
I realized that the original Kingsley pedals used these little Amvecos wired backwards (primary becomes the secondary and vice-versa) to go from 12 V to 250 V on the tubes (pretty ingenious IMO).
Then I noticed that this guy uses 2 toroids about the same size to accomplish the same thing: Alembic F-2B preamp | moosapotamus
Except he uses DC on his filaments. Personally, I'd like to get the 6.3 V another way. Seems to me like I could just use large resistors to drop it down (wirewound with heat sink or cement). How do I figure out how many watts to dump?
He shows Amveco 62072's (35 VA, 7 mA) but I have 62062's (25 VA, 5 mA).
I realized that the original Kingsley pedals used these little Amvecos wired backwards (primary becomes the secondary and vice-versa) to go from 12 V to 250 V on the tubes (pretty ingenious IMO).
Then I noticed that this guy uses 2 toroids about the same size to accomplish the same thing: Alembic F-2B preamp | moosapotamus
Except he uses DC on his filaments. Personally, I'd like to get the 6.3 V another way. Seems to me like I could just use large resistors to drop it down (wirewound with heat sink or cement). How do I figure out how many watts to dump?
He shows Amveco 62072's (35 VA, 7 mA) but I have 62062's (25 VA, 5 mA).
Simple ohms law - although you don't figure out 'how many watts to dump', you figure out the resistor value based on the current consumed and the voltage you need to lose.
However, assuming it's a 12V transformer?, then simply wire the valve for 12.6V heater.
Thanks Nigel, I didn't know that was possible! I've always seen 6.3V for heaters. Will it mean reducing the tube life? What other implications does it have?
I need to breadboard this power supply and will take actual measurements. I will also look up the current draw before I commit to anything. I'm still a newb but learning all the time.
Thanks very much for your input guys!
Aha I see you answered this already a few years ago:
http://www.diyaudio.com/forums/tubes-valves/75390-very-basic-12ax7-heater-question.html
Who knew? Well you did obviously...
What about a center tap? Yay or nay?
I need to breadboard this power supply and will take actual measurements. I will also look up the current draw before I commit to anything. I'm still a newb but learning all the time.
Thanks very much for your input guys!
Aha I see you answered this already a few years ago:
http://www.diyaudio.com/forums/tubes-valves/75390-very-basic-12ax7-heater-question.html
Who knew? Well you did obviously...
What about a center tap? Yay or nay?
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No implications at all, it makes no difference to the way it works.
However, it's only really the double-triodes which have the capability of doing this.
Fender has it. You can email them and request it or, even simpler, visit their website and download the file from Support / Amplifier Schematics -section.
Wow, I don't know why I never bothered to check there. Guess I assumed they would never post schems online but then this line was discontinued... Thanks for pointing it out! I made 11x17 copies and will compare to the Rumble schems. I'm confident I can pull this off now.
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